Zone annealing in dispersion strengthened materials

ABSTRACT

A MTHOD IS PROVIDED FOR PRODUCING A HOT WORKED HIGH TEMPERATURE HEAT RESISTANT DISPERSION-STRENGTHENED SUPERALLOY, CHARACTERIZED BY A METALLOGRAPHIC STRUCTURE CONSISTING ESSENTIALLY OF LARGE COARSE GRAINS HAVING A PREFERRED ORIENTATION RELATIVE TO THE AXIS OF WORKING, THE METHOD COMPRISING SUBJECTING A DISPERSION-STRENGTHENED ALLOY, E.G., SUPERALLOY, THE ZONE ANNEALING AT AN ELEVATED GERMINATIVE GRAIN GROWTH TEMPERATURE TO FORM COARSE GRAINS DISPOSED IN THE WORKING DIRECTION OF THE ALLOY AND TO ACHIEVE IMPROVED PROPERTIES.

July 17, 1973 c s ET AL 3,746,581

ZONE ANNEALING IN DISPERSION STRENGTHENED MATERIALS Filed Jan. 31, 1972 3 Sheets-Sheet 1 O o m N 2 o o N N 2 0%" -8 N 2 2 E o 0 O m E o o 1 1 1 l l X o o o 0 LO 0 IO 0 N :02 m r m m m u a u p u 17, 1973 R CAIRNS ETAL 3,746,581

ZONE ANNEALING IN DISPERSION STRENGTHENED MATERIALS Filed- Jan. :51, 1972 s Sheets-Sheet 2 FIG.2D

1973 R. L. CA|RNS ETAL 3,746,581

ZONE ANNEALING IN DISPERSION STRENGTHENED MATERIALS Filed Jan. 51. 1972 S Sheets-Sheet 5 Hot Zone Travel Direction FI'G.3

United States Patent Ofilice 3,746,581 ZONE ANNEALIN G IN DISPERSION STRENGTHENED MATERIALS Robert lLacock Cairns and John Stanwood Ben amin,

Suflern, N.Y., assignors to The National Nickel Company, Inc., New York, N.Y.

Filed Jan. 31, 1972, Ser. No. 221,979 Int. Cl. (321d 1/26; B221? 3/24 US. Cl. 1 .8-11.5 F 9 Claims ABSTRACT OF TIE DISCLOSURE A method is provided for producing a hot worked high temperature heat resistant dispersion-strengthened superalloy, characterized by a metallographic structure consisting essentially of large coarse grains having a preferred orientation relative to the axis of working, the method comprising subjecting a dispersion-strengthened alloy, e. g., superalloy, to zone annealing at an elevated germ 1nat1ve grain growth temperature to form coarse grams disposed in the working direction of the allloy and to achieve improved properties.

This invention relates to a powder metallurgy method for producing a preferred microstructure in a dispers onstrengthened heat resistant alloy, such as dispersionstrengthened age-hardenable superalloys and, in particular, to a method for producing hot worked dispersionstrengthened superalloy shapes from mechanically alloyed metal powder characterized by improved high temperature stress-rupture and creep properties.

RELATED APPLICATION In US. Pat. No. 3,591,362, granted July 6, 1971, which is incorporated herein by reference, a method is disclosed for producing a mechanically alloyed composite metal powder. In its broad aspects, the method comprises mixing a compressively deformable metallic powder with at least one other powdered material from the group consisting of a non-metallic material and another metallic material and dry milling the mixture under conditions of repeated mutual impact compression sufiiciently energetic to substantially reduce the thickness of at least the compressively deformable metallic constituents of the mixture and for a time sufiicient to produce non-pyrophoric Wrought composite particles which individually have substantially the composition of the mixture.

In a particular embodiment of that invention, a dry charge of attritive elements (e.g., nickel balls of plus 4 minus /2 inch average diameter) and a powder mass of predetermined composition is provided comprising a plurality of constituents, at least one of the constituents bemg a compressively deformable metal in an amount of at least by volume, with the remainder of the powder mass being at least one other constituent from the group consisting of a non-metal and another metal, the metals having a melting point of at least l000 K. The volume ratio of the attritive elements to the powder mass is at least about 4:1 and, more advantageously, at least about 10:1. The charge is then subjected to agitation milling, for example, in an attritor mill, under conditions in which a substantial portion of the attritive elements is maintained kinetically in a highly activated state of relative motion, whereby to cause the constituents to unite and form composite metal particles, the milling being continued until cold worked composite metal particles are produced characterized by markedly increased hardness (that is, the particles contain a substantial amount of stored energy) and further characterized by an internal structure in which the constituents are intimately inter- 3,746,581 Patented July 17, 1973 dispersed. Thus, when the particles, which, in a preferred embodiment, are heavily cold worked to reach substantially the saturation hardness of the system involved, are subjected to a diffusion heat treatment, the intmiately interdispersed constituents diffuse one into the other rather quickly to produce a homogenized matrix.

:In its more particular aspects, the method is applicable to the production of dispersion-strengthened superalloys having a matrix composition normally very difiicult to produce by conventional powder metallurgy techniques, including alloys falling within the range of about 5% to 35% or even up to 60% chromium, about 0.5% to 6.5% aluminum, about 0.5% to 5.6% titanium, up to about 15% molybdenum, up to about 20% tungsten, up to about 10% columbium, up to about 10% tantalum, up to about 3% vanadium, up to about 2% manganese, up to about 2% silicon, up to about 0.75% carbon, up to about 0.1% boron, up to about 1% zirconium, up to about 0.2% magnesium, up to about 4% hafnium, up to about 35% iron, up to about 10% by volume of a refractory dispersoid, and the balance essentially nickel in an amount at least about 40% of the total composition. As will be appreciated, cobalt can replace nickel. It is understood, therefore, that when nickel is mentioned herein, it is deemed that cobalt is an equivalent.

In copending application Ser. No. 131,761, filed Apr. 6, 1971, in the names of John S. Benjamin, Robert L. Cairns and John H. Weber and assigned to the same assignee, a process is provided by means of which coarse elongated grains can be uniformly produced substantially throughout a metal shape or member, having as its object to enhance still further the mechanical properties of hot worked shapes produced from composite metal powders of the type described in the previously mentioned US. Pat. No. 3,591,632. This is achieved in copending application Ser. No. 131,761 by providing a confined batch of cold worked mechanically alloyed composite particles comprised of alloying constituents which when alloyed together provide a superalloy composition (which is preferably age-hardenable) and then hot working (e.g., hot extruding) the confined batch under correlated conditions of temperature, reduction ratio and strain rate such that when the resulting hot worked product is subsequently heated to an elevated germinative grain growth temperature, e.g., from about 2250 F. to below the incipient melting point of the alloy for a time up to about 4 hours or more, coarse grains are formed disposed in the working direction of the alloy. For purpose of brevity, the subject matter of application Ser. No. 131,761 is similarly incorporated herein to the extent necessary to understand the background leading to the present invention.

OBJECTS OF THE INVENTION It is thus an object of this invention to provide an improved method for enhancing the high temperature-stressrupture and creep properties of dispersion-strengthened superalloys.

Another object is to provide a hot worked dispersionstrengthened superalloy shape having an improved metallurgical structure characterized by a substantially uniform distribution of coarse elongated grains disposed in the working direction of the alloy shape.

A further object is to provide a method of materially enhancing the mechanical properties of an extruded age-hardenable superalloy shape, such as age-hardenable dispersion-strengthened superalloys, by employing a relatively high temperature directional grain growth or annealing step.

Still another object is to provide a simple hot working and annealing or high temperature grain growth step for producing a dispersion strengthened age hardenable nickel-base superalloy shape having a substantially uniform distribution of coarse elongated grains across the cross section.

These and other objects will more clearly appear when taken in conjunction with the following disclosure and the accompanying drawing, wherein:

FIG. 1 is a plot on semi-logarithmic coordinates showing the relationship of extrusion ratio and extrusion temperature for hot working dispersion-strengthened nickelbase, age-hardenable superalloys, the area shown outlining the preferred combinations of extrusion ratio and extrusion temperature for subsequent application of the present invention;

FIGS. 2A through 2D are reproductions of a series of photomacrographs taken at 2 times magnification comparing the effects of conventional annealing (FIG. 2A) and of zone annealling (FIGS. 2B, 2C and 2D) on the size and shape of coarse grains obtained in dispersionstrengthened superalloy bars; and

FIG. 3 is a photograph of a partially zone annealed bar in section to illustrate the effect of zone annealing upon an extruded superalloy bar.

STATEMENT OF THE INVENTION Generally speaking, the present invention is directed to a method for producing a hot worked dispersionstrengthened heat resistant alloy, e.g., superalloy, shape characterized by improved mechanical properties at elevated temperatures and by a metallographic structure consisting essentially of large coarse grains disposed in the direction of working of the hot worked shape. One aspect of the invention resides in providing a batch of mechanically alloyed composite particles formed of constituents which, when alloyed together, provide a high temperature heat resistant alloy, e.g., an age-hardenable dispersion-strengthened superalloy, the composite particles being of substantially saturated hardness. The foregoing composite particles are characterized metallographically by an internal structure comprising said constituents substantially intimately united and interdispersed. A confined shape of the mechanically alloyed composite particles is hot worked in accordance with the invention at a temperature of over about 1600 F. and ranging up to about 2200 F. correlated to reduction ratios ranging broadly from over about 7 to about 35 (as more fully discussed hereinafter), and at a strain rate greater than a minimum value defined hereinafter such that when the resulting hot worked alloy is subsequently subjected to zone annealing at an elevated germinative grain growth or annealing temperature, elongated coarse grains are formed with the longitudinal axis thereof disposed in the working direction of the alloy shape. By zone annealing is meant the phenomenon by which coarse grains are formed by causing a hot zone at an elevated germinative grain growth temperature to traverse a hot worked shape from one end to the other in the direction of working so as to cause sequential growth of coarse elongated grains through the hot worked shape. For example, where the alloy shape is one obtained by extruding a cylinder of 3.5 inches in diameter to a rod three-quarters of an inch in diameter, the coarse grains are elongated like fibers in the direction of working, that is, in the longitudinal direction of the rod. Similarly, where the final extruded shape has a rectangular cross section, the coarse grains may be plate-like in shape, the major axis of each grain being generally disposed in the direction of Working.

The foregoing method is unique in that the internal stored energy required for grain growth is introduced by the mechanical alloying process in producing the initial powder and by the thermomechanical processing conditions. In the case of hot working by extrusion, a single hot extrusion is sufiicient to effect the consolidation of a product capable of developing coarse grains at an elevated temperature to provide a metallographic structure characterized by large coarse grains elongated in the di- 4 rection of working. By employing the aforementioned method, excellent high temperature properties are obtained in the grain grown product without the need of further working.

The foregoing zone annealing process is particularly advantageous over the grain growth technique described in copending application Ser. No. 131,761 in that the final product tends to exhibit additionally improved high temperature properties in combination with improved ductility as measured by percent elongation at failure of test specimens subjected to high temperature stress-rupture tests at various applied stresses. When grain growth is induced by a slowly moving hot zone traversing the hotworked consolidated shape, continuous growth of coarse grains in the direction of zone motion is generally favored. In this way, larger, more elongated grains are formed, and improved mechanical properties result.

In carrying out one embodiment of the method aspects of the invention, the moving hot zone was produced by means of a resistance furnace travelling on a carriage surrounding a stationary ceramic tube which contained the hot worked specimens or bars to be treated.

As will be understood by those skilled in the art, the moving hot zone may be produced in other ways, such as by means of moving induction coils or other heating elements; or, alternatively the specimen itself may be moved through a hot zone of a stationary furnace.

While the invention has been applied to the production of a dispersion-strengthened, age-hardenable nickel-base alloy having a nominal composition consisting essentially by weight of about 19% chromium, about 2.4% titanium, about 1.2% aluminum, about 0.07% zirconium, about 0.007% boron, about 0.05% carbon, and the balance essentially nickel, it is to be understood that the invention is applicable to dispersion-strengthened alloys in general. The dispersoid added to the composition, e.g., ThO Y O and the like, may be in the range of about 1% to about 5% or even up to about 10% by volume, e.g., nominally about 2.25 volume percent of particles preferably having average si-ze range of about to 500 or even about 1000 angstroms, e.g., about 300 angstroms. The foregoing superallo'y is the dispersion-strengthened, hot extruded condition exhibits improved high temperature stress-rupture properties when it is preferably subjected to a grain coarsening heat treatment by zone annealing, at a temperature of at least about 2200 F. and preferably at a temperature of at least about 2300 F. Thereafter, the alloy may be further heat treated and age-hardened. Prior to the germinative grain growth treatment by zone annealing, the foregoing is achieved by advantageously controlling in combination the hot working reduction ratio (e.g., the extrusion ratio), the hot working or extrusion temperature and the hot working strain rate. The coarse grains are characterized by preferred orientation after germinative grain growth treatment. In the case of an extruded rod-like product, there is an increase in grain size of at least 100 fold in the longest direction.

The reduction ratio is determined by the original cross section area of the shape before working divided by the cross section of the final product produced therefrom after working. For example, a shape or billet of 3.5 inches in diameter hot worked (e.g., hot rolled, hot press forged or hot extruded) to a final diameter of about five-eighths of an inch undergoes a change in cross section corresponding to a reduction ratio of about 31.4: 1.

It has been found that when extrusion is employed, the extrusion temperature, i.e., the temperature to which the material is heated for extrusion, for uniform results should advantageously not be less than about 17-60 F. and may range up to about 2200 F.

While further mechanical hot or warm working is permissible after the hot consolidation as by hot extrusion, it is not essential to the development of the desired microstructure.

An important advantage of the invention resides in the use of mechanically alloyed metal particles of substantially saturation hardness. By using such metal powder in the process of the invention, large coarse elongated grains can be produced uniformly across substantially the whole cross section of the final product. This is an unexpected improvement, considering that in normal extrusion processes, the grain size after recrystallization may be different across the cross section due to strain gradients varying from a maximum at the outside surface of the hot worked product to a minimum at the center thereof. The advantages achieved by the method will be appreciated from the following detailed description of the invention.

DETAIL ASPECTS OF THE INVENTION As stated hereinabove, it is important in achieving the results of the invention to employ in combination mechanically alloyed metal powder of substantially saturation hardness, to control the hot working reduction ratio (e.g., extrusion ratio), the hot working temperature, the hot working strain rate and to employ a grain coarsening heat treatment. All factors are important to obtain the desired results. Where hot extrusion is employed as the hot working technique, it has been found that the minimum ram speed should be that determined by the follow ing equation:

( K p (Q/ wherein V =ram speed D=diameter of the extrusion press liner (elfective extrusion billet diameter) =extrusion ratio T=extrusion temperature in K.

Q=65,000 calories per mole R=gas constant K=a constant ranging from 0.64 sec. to 6.4 X 10 sec. (preferably at least 2.175 10 /sec.)

E =thermomechanical energy component ranging from 1.793 to 2.250 (preferably at least 2.028)

It should be appreciated that, in extrusion, strain rate is not directly measurable. Strain rate in extrusion is known to be directly proportional to the speed of the extrusion ram and inversely proportional to the diameter of the press liner (billet container). The foregoing empirical Equation 1 is based upon data obtained on extruded bar produced in a 750-tone Loewy-BLH Hydropress extrusion press having a liner diameter of 3.5 inches and having the nominal composition set forth hereinbefore in the section entitled Statement of the Invention. For this press and alloy combination, the invention has been found particularly applicable to bars extruded within the closed area KLMNK of FIG. 1. Within this area the conditions of Equation 1 are met by the aforementioned press operated at full throttle. It should be realized that other presses which are capable of delivering higher strain rates would satisfy the requirements of Equation 1 over a wider region than KLMNK and produce material suitable for subjection to the process of the invention.

As illustrative of the method employed in achieving the results of the invention, the following example is given.

EXAMPLE 1 A nickel-titanium-aluminum master alloy is first prepared by vacuum induction melting. The resulting ingot is crushed and ground to minus 200 mesh powder. The powder (Powder A) contains 72.93% nickel, 16.72% titanium, 7.75% aluminum, 1.55% iron, 0.62% copper, 0.033% carbon, 0.050% A1 0 and 0.036 TiO About 14.9 weight percent of this powder is blended with 63.7% carbonyl nickel powder having a Fisher subsieve size of about 5 to 7 microns, 19.8% chromium powder having a particle size passing 100 mesh, 00.25% of a Ni-28% zirconium master alloy passing 200 mesh, 0 .04% of a Ni-17% boron master alloy passing 200 mesh and about 1.33% by Weight of yttria of particle size of about 350 A. About a 10,000 gram weight of powder blend is dry milled in an attritor mill using 10 gallons (about 390 pounds) of plus A inch carbonyl nickel pellets or balls, at a ball-to-powder volume ratio of about 18 to 1 ina sealed air atmosphere for about 20 hours with an impeller speed of 182 r.p.m. The time of milling is sufficient to produce wrought composite metal particles to and beyond the point of saturation hardness. Several batches of the powder are made by the foregoing method, the batches being thereafter sieved to remove abnormally large particles, for example, plus 45 mesh. The microstructure of the particles making up the powder is characterized by nearly complete homogeneity, when viewed optically at 250 diameters, comprising each of the constituents substantially intimately united and dispersed. With this material it was found that increasing the time of milling at 132 r.p.m. from 20 to 40 hours markedly improves homogeneity of the mechanically alloyed powder to the point that fragments of the starting ingredients become practically indistinguishable upon optical examination at 250 diameters. Experience indicates that in the aforementioned mill, the structural homogeneity obtained about 20 hours milling at 182 r.p.m. is about the same as that obtained upon 40 hours milling at 132 r.p.m.

In producing an extruded shape of the alloy, sufficient weight of the composite powder of minus 45 mesh is confined within a mild steel extrusion can having a diameter of about 3.5 inches. A plurality of billet assemblies was produced in this way and each assembly was then extruded at a full throttle setting on a Loewy Hydropress of 750 tons capacity using a hot graphite follower block behind the billet and at different reduction ratios and temperatures. At high temperatures and low extrusion ratios, the product was underworked and the grain structure appeared mixed, containing both fine and coarse grains, after the same germinative grain growth heat treatment. For application of the zone annealing process, we have found hot extrusion from temperatures of about 1600 F. to 2200 F. to be particularly advantageous when correlated to extrusion ratios as shown in FIG. 1. The correlation in FIG. 1 was determined experimentally in the aforementioned 750-tone Loewy Hydropress.

The coordinates at the intersections of the lines bounding the closed curve KLMNOK on FIG. 1 are as follows with extrusion ratio and extrusion temperature being given respectively in each case: K(7; 1600 F.); L(9.5; 1600 F.); M(35; 2020 F.); N(35; 2210 F.) and 0(7; 1700 F.).

High Temperature Zone Annealing In carrying out the invention, a number of extrusions was produced of various dispersion-strengthened nickelbase superalloys using 3.5 inch diameter billets of canned mechanically alloyed powders milled beyond the point of saturation hardness. Hot graphite follower blocks were used behind each billet. Stress-rupture properties were compared after a conventional heat treatment of 2 hours at 2325 F., 7 hours at 1975 F. and 16 hours at 1300" F., and after a heat treatment in which the first step of 2 hours at 2325 F. was replaced by the passage of a hot zone (2325 F.) about 2.2 inches long along the specimen at a rate of 1.75 inches per hour.

The nominal alloy compositions of the specimens extruded are given as follows:

TAB LE 1 Alloy number 0 7 The extrusion conditions are set forth in Table 2 below:

TABLE 2 Extrusion Temp., Ram speed,

Alloy number F. Ratio inches/see.

TABLE 3 Rupture Percent Alloy Heat Stress, Temp., li number treatment k.s.i. F. hrs. El RA 3 1 Conventional- 15 1, 900 3. 1 0. 8 16 1, 900 1. 8 0. 8 0. 9 17 1, 900 0. 9 0. 8 0. 9 40 1, 400 7. 1 0.8 0. 8

Invention...: 16 1,900 961 5. 6 10. 4 17 1, 900 324 5. 2 17. 9 19 1, 900 4. 8 11. 0 22. 0 50 1, 400 0. 2 41. 7 52. 2

2 Conventional- 14 1, 900 1. 5 1 0 45 1, 400 2. 8 1. 6 4. 5

Invention...... 15 1, 900 1, 800

3- Conventional- 13 1, 900 2.9 0 1. 2 15 1, 900 0.5 1 0 37. 5 1, 400 123. 5 4 1 40 1, 400 37. 7 0 0 Invention-.--- 15 1, 900 646 3. 2 4. 8 16 1, 900 142 4 7 40 1, 400 126. 7 6 13 45 1, 400 38. 7 5 12 3 Conventional- 13 1, 900 66. 9 0. 6 1 15 1, 900 5. 45 l. 3 0. 5 40 1, 400 198. 6 0.6 1. 4 45 1,400 32 4 0.6 0

Inventiom- 16 1, 900 686 1. 6 1 18 1, 900 14. 8 3. 2 5. 5 45 1, 400 41 4. 8 7. 4

1 K.s.i.=1,000 lbs/in. 2 El=percent elongation. 3 RA= percent reduction-in area:

It will be noted that the results obtained with the heat treatment of the invention are generally superior to those obtained with the conventional heat treatment in two respects: the stress-rupture lives are higher and the corresponding ductility as determined by percent elongation or percent reduction in area at failure is greater. The results in relation to Alloy No. 2 indicate that the procedure of the invention enabled raising the 100 hour rupture life at 1900 F. of this material from less than 15 k.s.i. as conventionally processed to greater than 17 'k.s.i. after zone annealing. Bars extruded under conditions near the line KON of FIG. 1 will be improved mainly in ductility as compared to conventionally annealed material.

In another instance canned mechanically alloyed powders containing about 0.05% carbon, about 19% chromium, about 1.2% aluminum, about 2.4% titanium, about 0.07% zirconium, about 0.007 boron 33% yttria, by weight, and the balance essentially nickel having a particle size of about 350 A., were extruded at 215-0 F. at an extrusion ratio of 31.4. In this instance, the extrusion ram speed was too slow especially in view of the high extrusion temperature. A throttle setting of only 30% for the 750-ton press was used and unsatisfactory properties were developed at 1900 F. even after zone annealing of'the resulting bar. Thus, a rupture life of 106 hours with an elongation of 16% and a reduction in area of 35% was determined at 1900 F. under an applied stress of 15 k.s.i.; and, at 1400 F., under an applied stress of 30 k.s.i., a rupture life of 1.5 hours with an elongation of 45% and a reduction in area of 62% resulted. The throttle setting for the extrusions in the case of each of Alloys 1 to 4 was on the 750-ton press.

As stated hereinabove, the moving hot zone was applied at a rate of 1.75 inches per hour. The actual heating rate and time at temperature of the bar being heated will depend on the size of the bar being heated, the size of the hot zone, the temperature profile of the hot zone, and the rate of relative movement between the hot zone and the bar being heated (one or the other may move). The required heating rate is easily obtainable by experimentation.

For example, the heating of the extruded bar to the germinative grain growth temperature should not be so slow in the range of about 1900" F. to 2200 F. (referred to as the sub grain-growth energy annealing range), that stress or energy annealing occurs to the extent that the stored energy in the work being treated is substantially removed. Stress or energy annealing is relatively slow at temperatures up to about 1900 F. However, the residence time during the heating up period above 1900 F. and below the grain growth temperature is important and should not be so long, e.g., in excess of 30 minutes, that the metal bar is substantially stress or energy annealed. When the residence time in the temperature region between 1900 F. and the grain growth temperature is too long, generally fine grains are obtained when the specimen involved is finally heated to the germinative grain growth temperature.

Thus, it is important that the heating rate through the temperature range below the germinative grain growth temperature should be sufficiently rapid to avoid substantial stress and energy annealing before the bar reaches the germinative grain growth temperature. On the other hand, the residence time of the alloy bar at the germinative grain growth temperature should be at least sufiicient to promote germinative grain growth. Tests have indicated that once the extruded alloy bar has reached the appropriate germinative grain growth temperature, the residence time should be at least about 5 minutes and, more preferably, in excess of about 10 minutes.

It is to be appreciated that the temperature reached at the center of the bar being heated lags behind the temperature imposed by the travelling hot zone. Furthermore, the maximum temperature of the bar being heated generally will be below the maximum temperature of the hot zone. In addition, if the zone is too short and/or travels too fast, the desired temperature may not be reached in the bar being heated or the time at the germinative grain growth temperature may be too short.

Several tests were made at various hot zone speeds on extruded alloys made from mechanically alloyed powder similar to that described in Example 1, using a hot zone length above 2300 F. of about 2.2 inches. The results of these tests are given in the following examples:

EXAMPLE 2 An extruded alloy bar No. 5 was produced having a diameter of about five-eighths of an inch from mechanically alloyed powders of substantially saturation hardness and having the following composition by weight: about 20.5% chromium, about 2.9% titanium, about 1.5% aluminum, about 0.06% zirconium, about 0.007% boron, about 0.07% carbon, about 1.23% Y O having a particle size of about 350 A. and the balance essentially nickel. The bar was extruded from a 3.5 inch diameter can of powder at a temperature of about 2000 R, an extrusion ratio of about 31.8, and a ram speed exceeding one inch per second. The extruded alloy was subject to zone annealing at various zone speeds and the following results obtained:

As will be noted from Table 4, the residence time at 2300 F. should preferably exceed about minutes.

EXAMPLE 3 This example illustrates the efiect of too slow a speed through the hot zone. Material from the same alloy bar as that of Example 2 was employed. The results obtained 10 the same as those of Example 1. Yttria of varying particle size in varying amounts was included in each of the charges. The resulting powders were packed in 3 /2 inch diameter metal cans and were hot extruded without evacnation of the cans. In each instance, the extrusion ratio was 22:1 and the heating temperature prior to extrusion was 2000" F. with the exception of Alloy 6 for which the heating temperature was 1950 F. Alloy 6 contained 2.25% by volume of yttria having a particle size of 162 angstroms, Alloy 7 contained 2.25 volume percent of yttria having a particle size of 170 angstroms. Alloys 8, 9 and 10 contained, respectively, 1%, 4% and 5.5% volume of yttria having a particle size of about 151 angstroms and Alloy 11 contained 5.5 volume percent of yttria having a particle size of 278 angstroms. Test bars made from each of the extrusions were subjected to conventional heat treatment at high temperature followed by aging seven hours at 1975 F. and 16 hours at 1300 F., while corresponding test bars from each extrusion were also subjected to a high temperature zone anneal using a are s follows: 20

a zone traveling at two inches per hour. Each of the thus TABLE 5 heat treated bars was subjected to stress rupture testing Calculated Tiigg pg t w g at 1900 F. with the results set forth in the following Zone Speed 6 13 5 i Tables 6 and 7 containing respectively the results of testinches/hour minutes Grain structure minutes mg conducted upon the conventionally heat treated mate- 920 Fmem, n 712 rial and the zone annealed material. The grain growth 224 Fine in center 17% temperature, or zone temperature, and the grain shape {22 55 f,' ;a;; 114 resulting from the high temperature heat treatment is 132 o 10 also given in each of the tables.

33 Coarse, elongated.-. 26 30 TABLE 6 It ill b vid t from the data of Table 5 that the Conventional Heat Treatment-1,900 F. Stress Rupture residence time in the temperature range between 1900 P. A Rqcmtam. Percent and 2200" F. should be less than 60 minutes. W v- Grain Stress Life.

number eratureF. sh k. The rate of travel of the hot zone may range from ape 81 hours Elons RA above 1 inch to 17 inches per hour, preferably from about 2.325 q ax d- 16 0 0.8 0.8 7. 2,325 --do 15 25 1.3 1.6 2 lnches to 15 inches per hour. It W111 be appreclated that g 2425 15 0,1 34 the upper speed limit is a function of bar diameter and '2 gig? .5 .3 g furnace characteristics. The important thing is to bring 1 :3 11: 15 1 0,9 the entire bar cross-section, including the center, to the TABLE 7 Zone Reerystullization-1,900 F. Stress Rupture IOU-hour Zone Percent stress tempora- Grain Stress Life, improveture,F. shape k.s.i. hours Elong. RA. ment, k.s.i.

Coarse 17 530 1. 8 2. 0 {mange/ted. 19 1.4 9.7 30 3 2 350 {Coarse 16 290 5.3 3.0 2

Elongated 2 400 {Coarse 18 230 3.2 11 5 Elongated-- 19 38.4 4.8 18 2 400 {Coarse 17 37 2.7 4 3 s as .2

OBISB 2 400 ed 19 4.5 10.4 29} 5 17 17 6.4 9 3 grain growth temperature in the time required and keep it at that temperatures for at least 5 to 10 minutes and not longer than 4 hours. It will be observed from this example that the temperature profile of the hot zone should be as steep as possible to minimize residence time of the work being heated at lower temperatures, e.g., below about 2150 F., caused by zone temperature profile, metal conductivity and other eifects.

Regardless of the particular temperature profile employed, the residence time for germinative grain growth at temperatures ranging from 2150 F. to below the incipient melting point of the alloy should be in excess of 5 minutes, preferably at least about 10 minutes, and range 7 up to about 4 hours.

EXAMPLE 4 Broadly speaking, the zone annealing heat traetment may comprise at least a first step by zone annealing at an elevated annealing temperature to solution treat, homogenize and germinatively grow the grains and form large coarse grains in the direction of working of the hot worked product. In certain cases, a resolution treatment may be required because of coarse gamma prime precipitation occurring during slow cooling from the high zone annealing temperatures. Subsequent heat treatment to age the alloy to the desired hardness and strength may or may not be employed. Aging may not be required where the alloy is used at a temperature at which aging occurs in situ. Aging treatments comprise one or more steps and may be a series of aging sub-steps at successively lower temperatures where desirable. Thus, for an alloy comprising nominally the preferred. composition set forth hereinbefore, a three-step heat treatment found particularly advantageous comprises: (1) subjecting the hot worked alloy to a grain growth temperature of about 2325 F. to 2400 F. by means of a moving hot zone, wherein a protective environment, e.g., argon, may be provided; (2) thereafter, heating at a resolution temperature of 1975 F. for a suitable time, e.g., 7 hours, in air followed by air cooling; and (3) finally aging the alloy at 1300 F. for a longer time, such as 24 hours, in air and then air cooling. The first step results in a marked increase in grain size by zone annealing having a preferred orientation relative to a working direction. For example, as stated hereinbefore, in the case of an elongated extruded product, the coarse gra-ins are elongated and are disposed or exhibit a preferred orientation in the direction of extrusion, that is, the longitudinal axis of the elongated product. In the case of a hot worked product in which the cross section is somewhat rectangular, the grains tend to be plate-like and to be disposed or show a preferred orientation in the direction of the major axis of working, that is, in the longitudinal direction and thus exhibit higher mechanical properties in the longitudinal direction.

The coarse grains generally exhibit aspect ratios of greater than about 3:1, in some cases greater than 10:1 or :1 or even higher. The aspect ratio is that ratio that defines grain configuration correlated to the direction of interest, e.g., direction of applied stress. The ratio is determined as the average dimension of the grain parallel to the direction of interest divided by its average dimension along a minor axis.

Commensurate with the formation of the coarse grain structure is an incremental improvement of the stressrupture properties at both intermediate temperatures, e.g., 1400" F., and at high temperatures, e.g., 1900" F., determined in the direction of working.

FIG. 2 illustrates photomacrographs (A to D) taken at 2 times magnification showing the improved grain structures (after zone annealing heat treatment at 2325 F.) obtained on an alloy comprising by weight 18.2% chromium, 1.3% total aluminum, 2.3% titanium, 0.004% boron, 0.06% zirconium, 1.34% Y O 0.055% carbon, 0.68% total oxygen, and the balance essentially nickel. The alloy was produced from mechanically alloyed powder of substantially saturation hardness provided as described in Example 1, which powder was then placed into mild steel cans of about 3.5 inches in diameter. The cans were welded shut to form extrusion billets. The billet assembly was extruded at an extrusion speed exceeding one inch per minute using hot graphite follower blocks from the aforementioned diameter of about 3.5 inches through a die 0.625 inch in diameter corresponding to an extrusion ratio of about 31.4 to 1 at a temperature of about 2150 F. FIG. 2A depicts the kind of macrostructure obtained when the extruded bar is soaked conventionally at the germinative grain growth temperature of about 2325 F. for 2 hours, while FIGS. 2B, 2C and 2D show the microstructure obtained after zone annealing at the same temperature. With regard to FIG. 2B, the hot zone traversed the bar at about 4.4 inches per hour. This specimen was pointed at the starting end to encourage the formation of a limited number of grains. However, the comparison of FIG. 2B with FIG. 2C zone recrystallized under the same conditions showed that the pointing was not necessary. The specimen of sample FIG. 2D was pointed and then cold deformed at the point and the hot zone then allowed to traverse the sample at a rate of 1.75 inches per hour. Fewer grains formed at this rate of travel. FIG. 3 illustrates a par produced similarly to those of FIGS. 2A through 2D which was removed from the furnace and cooled before complete passage of the hot zone which traveled at 18.3 inches per hour. The fine grain of the untreated material is clearly evident as is the effect of advancing the hot zone upon the grain structure in the zone annealed portion of the bar. The effect of the temperature lag in the center of the bar as compared to the bar surfaces is also shown. The capability of producing parts which are fined-grained in one portion and 12 coarse-grained in another portion is clearly demonstrated by FIG. 3.

Stating it broadly, the heat treatment of hot worked alloy products produced from mechanically alloyed powder may vary over the following ranges:

First step: Zone annealing from about 2200 F. to below the incipient melting point as described hereinabove; Optional second step: Solution heating at about 1750 F.

to 2400 F. for about /2 hour to 16 hours; Optional third step: And then aging at about 1150 F. to

1600 F. for about 1 to 100 hours.

While the invention has been described in conjunction with a nickel-base alloy, the invention is particularly applicable to the following range of compositions: about 5% to 60% chromium, about 0.5% to 6.5% aluminum; about 0.5% to 6.5% titanium; up to about 15% molybdenum, up to about 20% tungsten; up to about 10% columbium; up to about 10% tantalum, up to about 3% vanadium; up to about 2% manganese; up to about 2% silicon; up to about 0.75% carbon; up to about 0.1% boron; up to about 1% zirconium; up to about 0.2% magnesium; up to about 6% hafnium; up to about 35% iron; up to 10 volume percent of a dispersoid, the balance essentially atleast about 40% nickel. Generally speaking, the alloys to which the invention is applicable have a melting point of at least about 2300 F. The composition may comprise cobalt since nickel is generally considered an equivalent of cobalt. The dispersoid may include those selected from the group consisting of ThO Y O La O and the rare earth mixtures didymia and Rare Earth Oxides, and other oxides having free energies of formation exceeding kilocalories per gram atom of oxygen at about 25 C. The size of dispersoid found advantageous in producing dispersion-strengthened superalloys may range from about 50 angstroms to 5000 angstroms, and, more advantageously, from about angstroms to 1000 angstroms. Preferably, the alloys contain about 10% to 35% chromium, about 0.5% to 6% aluminum, about 1% to 5% titanium, up to about 5% molybdenum, up to about 10% tungsten, up to about 3% columbium, up to about 5% tantalum, up to about 15 cobalt, up to about 1% vanadium, up to about 2% manganese, up to about 1% silicon, up to about 0.2% carbon, up to about 0.1% boron, up to about 0.5% zirconium, up to 0.2% magnesium, up to 2% hafnium, up to about 10% iron, about 0.5 volume percent to 5 volume percent of a dispersoid, the balance essentially at least about 40% nickel.

The product provided in accordance with the invention is useful in the production of articles such as gas turbine blades and vanes and other articles subjected in use to the combined effects of elevated temperature and stress.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modification and variations may be resorted to without departing fromthe spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. In the method for producing a hot worked dispersion-strengthened superalloy shape having improved properties at elevated temperatures wherein mechanically alloyed powders of superalloy composition and of substantially saturated hardness are hot worked to form a consolidated superalloy shape under coordinated conditions of temperature, reduction ratio and hot working strain rate such that when the resulting consolidated shape is heated to a germinative grain growth temperature large grains result, the improvement which comprises subjecting said hot worked consolidated superalloy shape to zone annealing by passing a hot zone maintained at an elevated germinative grain growth temperature along said hot worked shape, the heating rate to said germinative grain growth temperature being sufficiently rapid to avoid substantial strain annealing, said consolidated article being heated to a germinative grain growth temperature along successive portions thereof such that successive portions of aritcle are heated at the germinative grain growth temperature for at least minutes.

2. The method according to claim 1 wherein said germinative grain growth temperature is between about 2200 F. and the incipient melting point of said superalloy.

3. The method of claim 1, wherein the heat resistant alloy has a composition ranging by weight from about 5% to 60% chromium, about 0.5 to 6.5% aluminum, about 0.5% to 6.5% titanium, up to about 15% molybdenum, up to about 20% tungsten, up to about columbium, up to about 10% tantalum, up to about 3% vanadium, up to about 2% manganese, up to about 2% silicon, up to about 0.75% carbon, up to about 0.1% boron, up to about 1% zirconium, up to about 0.2% magnesium, up to about 6% hafnium, up to about 35 iron, an eifective amount of up to about 10% by volume of a refractory dispersoid, and the balance essentially a metal from the group consisting of nickel and cobalt in an amount at least about 40% of the total composition, the alloy having been hot worked under conditions of temperature and reduction ratio lying within the closed area of FIG. 1, the residence time at the germinative grain growth temperature being substantially in excess of 5 minutes.

4. The method of claim 3, wherein the composition ranges from about 10% to 35% chromium, about 0.5% to 6% aluminum, about 1% to 5% titanium, up to about 5% molybdenum, up to about 10% tungsten, up to about 3% columbium, up to about 4% tantalum, up to about cobalt, up to about 1% vanadium, up to about 2% manganese, up to about 1% silicon, up to about 0.2% carbon, up to about 0.1% boron, up to about 0.5%

14 zirconium, up to 0.2% magnesium, up to 2% hafnium, up to about 10% iron, about 0.5 volume percent to 5 volume percent of a dispersoid, the balance essentially at least about nickel.

5. The method of claim 4, wherein the chromium content of the alloy ranges up to about 25%.

6. The method of claim 1, wherein the residence time at the germinative grain growth temperature is at least about 10 minutes.

7. The method of claim 1, wherein following germinative grain growth the alloy is heated to a solution temperature of about 1750 F. to 2400 F. for about 0.5 hour to 16 hours and thereafter aged at a temperature ranging from about 1 150 F. to 1600" -F. for about 1 to 100 hours.

8. The method of claim 4, wherein the alloy comprises about 19% chromium, about 2.4% titanium, about 1.2% aluminum, about 0.07% zirconium, about 0.007% boron, about 0.05% carbon, about 2.25% by volume of a dispersoid and the balance essentially nickel, and wherein said alloy is subjected to germinative grain growth by zone heating it to a temperature of at least about 2300" F. but below the incipient melting point of the alloy for up to about 4 hours.

9. A zone annealed high temperature heat resistant dispersion-strengthened alloy produced in accordance with the method of claim 1.

References Cited UNITED STATES PATENTS 3,388,010 6/1968 Stuart et al. -05 BC 3,459,546 8/ 1969 Lambert 750.5 BC 3,524,744 8/1970 Parikh 75-05 BC WAYLAND W. STALLARD, Primary Examiner US. Cl. X. R. 148126 222g? UNITED 'STATESPATENT OFFICE e @ERTIFICATE OF CORRECTION Patent No. 1 1 1 Datcd July 17, L223 I ROBERT LACOCK CAIRNS and JOHN STANWOOD BENJAMIN- It is certified that error appears in the above-identified patent and that: said Letters Patent are hereby corrected as shownbel owz In the heading, for "National" read ln't ernational- Col. 2, line 4, for "intrniately read --inti mately--- 'Line 13, for "5.5%" read --6.5%--. Col. 4, line 42 for "is" read "in- I Line for by"- read -as' Col. 5, line 48, for "tone" read -ton- Line 75, for "-00.25%" read ---o.25%-

Col 6, lin 45', for "tone?" read ;--ton-". C01. 7; line 68, fer "33%" reed -aebo u t i 33%-- Col a, line 70, for. "1.23%" read "-1.32%".

Claim 1, line 19, for Par-tale" read article Signd and sealed this 22 day of Jenuar 19m.

(SEAL) Attest:

EDWARD MQFLETQHERJR. RENE D. TEGTMEYER Attegt ing Qfflcer Acting Cpmmiss ioner of Patents;

553 V UNITED S ATESMTENT OFFICE EERTIFICAT-E OF CORRECTION patent 3,746,581 Dated Julv 17 1973 Inventor 5 ROBERT LACOCK CAIRNS and JOHN STANWOOD BENJAMIE are in the above-identified patent It is certified that error appe h orrected as shown below:

and that amid Letters Patent are hereby c I .1 In the heading, for "National" read InirernationaL-m Col, 2, line 4, for "intlniately read -intimately- Line 13, for "5 6%" read "6.5%". Col. 4, line 42, for "is" read "in- I I Line 59, fo r "by"-"read --as'-.

, Col. 5, line 48, for "tone read -ton---.v

Line 75, for "-00.25%" read --o.25%-- Col 6, line 45', for "toneY' read --ton-- Col. 7, line 64, fer "33%" read dbdur i 33%- Col. 8, line 70, for. "1.23%" read "1.32%".

C91" 1. 4 e f' ar?. i dqz-mrbarr v .4 a

Claim 1, line 19, for "artcle" read article Signed and sealed this 2 4 day of 'Jenuer 197d.

(SEAL) Attest:

EDWARD MJLETQHER, JR. RENE 1 TEGTMEYER I Atteetj ng Offlcer I Acting Commissioner of Petent's I 

